It is known that wire containing superconductor powders can be formed using powder-in-tube (PIT) technology. After the wires are drawn, they must be heat treated to melt the superconductor powder, which, upon cooling, can form superconductor crystals. The superconductor crystals can form continuous superconductor filaments in the wire.
A typical wire has one or more cavities for accommodating the superconductor powders, but each cavity can only accommodate an amount of superconductor powder equal to about ⅔ of the volume of each cavity. As a result, the heat treatment needed to melt the superconductor powders usually results in gas-filled internal voids within the filaments, the presence of contaminants, such as carbon and H2O, or a combination thereof.
The gas-filled internal voids commonly agglomerate into large, filament cavity-sized, gas-filled bubbles that can block current transport and/or lead to wire expansion and cracks during the heat treatment that melts the superconductor powders. Such effects are explained in the relevant literature (e.g., Kamentani, F. et al. SUPERCOND. SCI. TECHNOL., vol. 24, no. 7, p. 075009, July 2011; and Malagoli, A. et al. SUPERCOND. SCI. TECHNOL., vol. 26, no. 5, p. 055018, May 2013). As a result, the final processed wire typically has low critical current density, likely due to the small connected current paths within each filament.
Efforts to overcome one or more of these difficulties has resulted in a technique commonly referred to as overpressure (OP) processing (see, e.g., Larbalestier, D. C. et al. NAT. MATER., Vol. 13, No. 4, pp. 375-381, April 2014; U.S. Pat. No. 6,555,503; and WO 2001/022436). In typical OP processes, a wire, usually a coiled wire, is heat treated under sufficient isostatic pressure to compress the wire, thereby decreasing the filament porosity. The filament porosity may be decreased significantly, and, in some instances, to nearly zero in certain OP processes. As a result, the OP process can eliminate large bubbles, increase the mechanical and physical connectivity, and increase the critical current density.
However, the OP processes also reduce the diameter of the wire, typically up to about 5%. Due at least to the reduction in diameter, the coil packing density also can be reduced, which may cause relative movement of the turns of the coil, sagging, other displacements, or a combination thereof that may negatively affect the field quality. It has been discovered that OP processing can cause the top plane of a long coil to sag by about 5% after OP processing. Due to the changes that can be imparted to coils of wire, OP processing typically results in a loose winding pack that can undermine the predictability of coil geometry and coil uniformity. These changes also can be difficult, if not impossible, to correct, due at least to the brittle nature of certain superconductor wires.
Several technologies have been developed in an attempt to reduce the reduction in wire diameter that occurs during OP processing. These technologies include cold isostatic pressing (CIP), swaging, and rolling, which are performed at room temperature.
The CIP technology consists of using isostatic pressure at room temperature to compress a metal wire. Despite relatively high and commercially unfeasible pressures, however, the reduction in diameter that occurs during CIP is not significant enough to offset the foregoing problems caused by OP processing. Swaging involves mechanically hammering a metal wire between circular jaws to decrease its diameter (see Jiang et al. IEEE TRANS. APPL. SUPERCOND. 23, 3, 2013, 64002006-6400206). This process typically deforms the wire's internal architecture and can damage the cavities within the wires. Rolling involves the mechanical deformation of the conductor between two rolls. This process typically partially compresses the conductor by deforming a round wire into a tape, which is not the geometry usually preferred when building coils (see Miao, H. et al. PHYS. C SUPERCOND. 301, 1-2, 1998, 116-122; U.S. Pat. Nos. 6,694,600; and 6,632,776). Round rolling processes, like cassette rolling, can be similar to swaging. None of these processes, however, sufficiently address the disadvantages associated with diameter reduction that occurs during OP processing.
Therefore, methods that overcome one or more of the foregoing difficulties associated with powder cavity capacity and/or OP processing are desirable.